Alloy 6061 is a general-purpose structure alloy
to be permitted to come in contact with each other.
using a combination of magnesium and silicon as the
Observing these precautions enables routine
chief alloying ingredients. Its use in the Navy should
maintenance to be kept to aminimum.
be restricted to auxiliary systems, such as piping and
Galvanic corrosion caused by a dissimilar metal
railings, and to nonwelded structures.
contacting aluminum can occur. In marine
applications, aluminum and its alloys are frequently
Two other alloys that may be found in limited
the anodic metal and could corrode in preference to
quantities are 5083 and 7039. These are used only for
most other common contacting metals except zinc and
armor and are supplied especially for that purpose. As
magnesium. However, for galvanic corrosion to occur,
such, they should not be used for other structural areas
the following conditions must be satisfied: a cell must
of an aluminum boat.
be present consisting of at least two metals having
Aluminum alloys are not identifiable by
different solution potentials and in electrical contact
appearance and, therefore, are usually appropriately
with each other (no matter how indirect), and a
marked with alloy and temper designations. The
conductive medium (electrolyte) must be present
temper designation follows the alloy number and
between the metals.
indicates the degree of tempering. Tempering is done
Three applications account for most galvanic
in two ways depending on the alloy: either by strain
corrosion situations: (1) connections of aluminum
hardening or by the heat-treatment process. An alloy
deckhouse bulkheads to a steel boundary bar; (2) the
that has been strain hardened has a designator
attachment of steel or brass fittings to an aluminum
consisting of the letter H and a number, while an alloy
structure; or (3) dissimilar metal appendages, such as
that has been heat treated has a designator consisting
of the letter T and a number. Thus, a plate labeled
5086-H116 has been strain hardened, while one
Cleanliness is always important--dirty, wet bilges
marked 6061-T6 has been heat treated. Any alloy will
or accumulations of dirt and water anywhere are to be
be one or the other; for example, all tempers of 5086
avoided. A freshwater rinse on a regular basis is
begin with H. The exception is when aluminum is in
generally sufficient.
the soft, or annealed, condition--indicated by the
suffix 0. Thus, both 5086-9 and 6061-9 (and others)
ALUMINUM BOAT REPAIR
are available. The temper of material is of concern to
the repairer, since it is desirable to make replacements
Cutting aluminum is more like cutting wood than
of damaged areas with the same alloy and the proper
steel. An oxyacetylene flame is not used because the
temper.
heat away too fast to get a good cut. In repair work, all
Aluminum is a lightweight material, and it is for
cutting should be done mechanically using a circular
this reason that it is used for boats and craft. It is
saw, saber saw, or (in the shop) bandsaw equipment
strong, weldable, and has excellent general corrosion
with metal-cutting blades. Use of a grease stick or lard
resistance when proper marine alloys are employed.
oil will prolong blade life. Plasma arc cutting
In the past, most interior spaces of naval boats were
equipment is available for high-speed production
left unpainted in aluminum construction. There are,
work but is not needed for repair work. Shearing or
however, some precautions in the handling of
punching of strained hardened alloys should be
aluminum that must be observed if the full corrosion
avoided.
resistance capability of aluminum is to be achieved.
Forming is done cold or hot. Aluminum does not
As with many materials, although mild acidic
change color with heat and does not glow red as does
solutions cause slight damage, caustic solutions of any
steel. Excessive heating can cause the metal to anneal
sort such as sodium hydroxide, sodium carbonate, or
to the soft condition or even melt or oxidize without
sodium phosphate are particularly to be avoided; they
any warning. Hot forming is done by carefully heating
cause severe etching of the aluminum even to the
the metal to no more than 450F. The temperature can
extent of resulting in perforation.
be estimated by the use of temperature-sensitive
The most stringent precautions must be taken in
crayons. Each crayon is formulated to melt at a
the case of mercury. The presence of mercury even in
different temperature; and by observing when the
small amounts on aluminum causes severe corrosive
crayon markings on the metal melt, you can remove
attack, and under no circumstances are the two metals
the heat source at the proper time.
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